Recently, the display performances of liquid crystal displays (LCDs) have been improved to the point that more and more manufacturers adopt LCD panels as TV monitors, for example. As a result of those researches and developments, the viewing angle characteristic of LCDs has been improved to a certain degree but not satisfactorily in some respects. Among other things, there is still a high demand for improvement of the viewing angle characteristic of an LCD using a vertical alignment liquid crystal layer (which is sometimes called a “VA mode LCD”). As used herein, a “vertical alignment liquid crystal layer” refers to a liquid crystal layer in which liquid crystal molecules are aligned so as to have their axis form an angle of (i.e., have a pretilt angle of) approximately 85 degrees or more with respect to the surface of a vertical alignment film. The liquid crystal molecules have negative dielectric anisotropy and the VA mode LCD is combined with polarizers that are arranged as crossed Nicols to conduct a display operation in a normally black mode.
A VA mode LCD, which is currently used for a TV set with a big screen, for example, adopts a multi-domain structure in which multiple liquid crystal domains are formed in a single pixel region, to improve the viewing angle characteristic. An MVA mode is often adopted as a method of forming such a multi-domain structure. Specifically, according to the MVA mode, an alignment control structure is provided on one side of the two substrates, which face each other with a vertical alignment liquid crystal layer interposed between them, so as to face the liquid crystal layer, thereby forming multiple domains with mutually different alignment directions (i.e., tilt directions), the number of which is typically four. As the alignment control structure, a slit (as an opening) or a rib (as a projection structure) may be provided for an electrode, thereby creating an anchoring force from both sides of the liquid crystal layer.
If a slit or a rib is adopted, however, the anchoring force will be applied onto liquid crystal molecules non-uniformly within a pixel region because the slit or rib has a linear structure unlike the situation where the pretilt directions are defined by an alignment film in a conventional TN mode LCD. As a result, the response speed may have a distribution unintentionally. In addition, since the transmittance of light will decrease in the areas with the slits or ribs, the luminance on the screen will decrease, too.
To avoid these problems, the VA mode LCD also preferably has a multi-domain structure by defining a pretilt direction with an alignment film.
Examples of VA mode LCDs with an alignment control structure for controlling the pretilt directions with an alignment film include a VAECB (vertical alignment electrically controlled birefringence) mode LCD (see Patent Document No. 1, for example), an RTN (reverse twisted nematic) mode LCD and a VATN (vertical alignment twisted nematic) mode LCD (see Patent Documents Nos. 2 to 5, for example).
In the VAECB mode LCD, the pretilt directions of liquid crystal molecules, which are defined by two alignment films that face each other through a liquid crystal layer, are antiparallel to each other in an arbitrary domain in a pixel. That is why the tilt direction of liquid crystal molecules around the middle of the thickness of the liquid crystal layer agrees with the pretilt direction defined by the alignment film on the lower substrate. In this case, the tilt direction of those liquid crystal molecules is an azimuthal direction indicated by the arrow that represents the tip of the liquid crystal molecules pointing toward the viewer when the LCD is viewed by the viewer. Also, the tilt directions of the liquid crystal molecules are constant irrespective of the position in the thickness direction of the liquid crystal layer or the magnitude of the applied voltage.
On the other hand, in the RTN mode, the pretilt directions of liquid crystal molecules that are defined by the two vertical alignment films in an arbitrary domain of a pixel are substantially perpendicular to each other. Also, in the RTN mode, when a sufficiently high voltage (which is at least equal to a signal voltage to display the highest gray scale) is applied to the liquid crystal layer, liquid crystal molecules, located around the center of a plane of the liquid crystal layer and around the middle of the thickness of the liquid crystal layer, are tilted in a direction that substantially equally divides the two pretilt directions defined by the alignment films into two.
Examples of known methods for getting the pretilt directions defined by vertical alignment films include a process that requires a rubbing treatment and a process that requires an optical alignment treatment. Among other things, the optical alignment treatment can be done on the alignment films without making any physical contact with them. That is why unlike the rubbing treatment, the optical alignment treatment would produce no static electricity and could increase the yield. Furthermore, as disclosed in Japanese Patent Application No. 2005-141846 that was filed by the applicant of the present application, by using an optical alignment film including a photosensitive group that can form a bond structure, the variation in the pretilt angle can be reduced to one degree or less. As a result, the display luminance characteristic can be improved.
However, the optical alignment treatment has its own problems.
For example, Patent Document No. 6 discloses a technique for reducing the optical deterioration of an optical alignment film by arranging a member that absorbs UV rays between the optical alignment film that is located closest to the viewer and the uppermost surface for the viewer.
On the other hand, Patent Document No. 7 discloses a technique for minimizing the disturbance in alignment treatment that would be caused by light reflected from a taper portion of a metallic electrode, which is provided to produce a lateral electric field in a pixel, in a situation where the optical alignment treatment is adopted to fabricate an IPS mode LCD.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2001-281669        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 11-352486        Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 2002-277877        Patent Document No. 4: Japanese Patent Application Laid-Open Publication No. 11-133429        Patent Document No. 5: Japanese Patent Application Laid-Open Publication No. 10-123576        Patent Document No. 6: Japanese Patent Application Laid-Open Publication No. 2001-272682        Patent Document No. 7: Japanese Patent Application Laid-Open Publication No. 2005-128359        